Polymer grade terephthalic acid is the starting material for polyethylene terephthalate (PET), which is the principal polymer for polyester fibers, polyester films, and resins for bottles and the like containers. Polyester fibers are used in textiles as well as in industrial applications such as tire cord. Polyester films coated with adhesives and emulsions are useful as wrapping tapes, photographic films, recording tapes, and the like.
Polymer grade terephthalic acid is derived from relatively less pure, technical grade terephthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst as described in U.S. Pat. No. 3,584,039 to Mexer. In the purification process, the impure terephthalic acid is dissolved in water at an elevated temperature, and the resulting solution hydrogenated, preferably in the presence of a hydrogenation catalyst, e.g., palladium on a carbon support, as described in U.S. Pat. No. 3,726,915 to Pohlmann. This hydrogenation step converts the various color bodies present in the relatively impure terephthalic acid to colorless products. The principal feedstock impurity, 4-carboxybenzaldehyde, is converted to p-toluic acid.
The resulting purified product, polymer grade terephthalic acid, is recovered by crystallization, centrifugation, and drying. Another related purification-by-hydrogenation process for aromatic polycarboxylic acids produced by liquid-phase catalytic oxidation of polyalkyl aromatic hydrocarbons is described in U.S. Pat. No. 4.405,809 to Stech et al. Purification of terephthalic acid by modulating hydrogen concentration is disclosed in my U.S. Pat. No. 4,626,598 granted on Dec. 2, 1986 and incorporated herein by reference.
However, the variable nature of the impure terephthalic acid feedstock makes process control, and thus quality assurance, difficult and costly. To this end, it would be desirable to effect hydrogenation of an aqueous, impure terephthalic acid solution under conditions that optimize the reduction of specific colored compounds identified as causing the yellow color of improperly purified terephthalic acid. The present invention provides a convenient method for reducing specific colored aromatic compounds which cause the color problems in terephthalic acid.